Liquid scintillation counters are commonly used for measuring count rates or activities of radioactive samples containing beta, or corresponding, particle emitting radionuclides, e.g., H-3, C-14, S-35, 1-125, P-32 etc.
Many beta particles are of low energies and have ranges generally less than few tens of micrometers in the sample. As a consequence, the sample to be measured has to be intimately contacted with a scintillation medium, most often by dissolving it into a liquid scintillator which typically comprises an organic solvent and at least one fluorescent solute present in a few percent by weight of the solution. In radiation interaction processes, kinetic energy of the beta particle is absorbed by the solvent and then transferred to the solute which emits a burst of scintillation photons, whose amount is proportional to the energy of the beta particle. These scintillation photons are detected with a detector that usually consists of two, coincidence operating, photomultiplier tubes producing electric pulses. The height of the pulses are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle. Coincidence operation is employed for eliminating the random thermal noise pulses of photomultiplier tubes.
Normally liquid scintillation counters are provided with one detector and they are designed to measure samples in 7 ml or 20 ml glass or plastic vials.
A novel liquid scintillation counter, designed for counting samples directly from multi-well sample plates is shown in U.S. Pat. No. 5,061,853 (Lehtinen et al.), which apparatus can count liquid scintillation or corresponding samples directly from a sample plate which comprises several separate sample wells or vials. The apparatus has one or several detectors in order to count one or several samples at a time. The sample plate is placed in the counting position on a rigid plate holder made of photon attenuating material and having holes for the wells of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two photomultiplier tubes operating in coincidence and situated on the opposite sides of the holes of the plate holder. The wells of the sample plate can be closed by an adhesive transparent tape. The apparatus can be used also for counting gamma radiation emitting samples if the holes of the sample plate are surrounded by gamma radiation sensitive detectors.
Another novel scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate is presented under European Patent Publication Number 0425767A1 (VanCauter et al.). This apparatus is provided with multiple photomultiplier tubes positioned adjacent to the sample wells containing the scintillator for simultaneously measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses. The electrical pulses from each photomultiplier tube are processed to discriminate between pulses attributable to sample events within the wells and pulses attributable to non-sample events such as photomultiplier tube noise. The discrimination is effected by determining whether a selected number of electrical pulses occurs with a prescribed time interval, the occurrence of the selected number of pulses within the prescribed time interval signifying a sample event. Only the electrical pulses attributable to sample events are supplied to a pulse analyzer.
The multi-well sample plates have typically eight rows of wells, whose diameter is 7-8 mm arranged in twelve columns with 9 millimeters distance between the center points of the wells. The typical volumes of sample wells of such 96-well sample plates are 200-400 microliters depending on the height of the plate. Another type of multi-well sample plate has four rows of wells arranged in six columns. The volume of the wells of such 24-well sample plates exceeds one milliliter. When the wells of the multi-well sample plate are separate, the plate can be placed for counting on a rigid sample plate holder made of photon attenuating material and having thru-holes for the wells of the sample plate as shown in U.S. Pat. No. 5,061,853 (Lehtinen et al.). As a consequence, an optically isolated compartment is formed around each sample well of the sample plate. Unfortunately, most of the commercially available multi-well sample plates are transparent and the wells are joined together with ribs, bridges and the like in order to stiffen the sample plate. Therefore, it is impossible to place them in the above mentioned sample plate holder to isolate them optically. Consequently, some amount of the scintillation photons produced by the absorption of the beta particle in a certain sample well may travel to other sample wells and thus produce an undesired increase in observed count rates in those wells. This phenomenon is called optical crosstalk. It is known that the use of opaque multi-well sample plates can reduce optical crosstalk as mentioned in TopCount Topics PAN0005 6/91, published by Packard Instrument Company, Meriden USA 1991. However, in many applications transparent multi-well sample plates are preferred, and most of the commercially available multi-well sample plates are transparent.